Polarization independent optical isolator

ABSTRACT

A polarization independent optical isolator for transmitting a signal ray in a forward direction and suppressing backward transmission comprises: a first and second optical isolation unit for receiving, transmitting, and outputting signal rays in the forward direction and for suppress backward transmission of any rays; and a reflection unit for coupling to first and second optical isolation unit by directing the outputted signal ray from the first optical isolation unit to the second optical isolation unit, the signal ray received by the first optical isolation unit transmitting in the opposite direction to the outputted signal ray from the second optical isolation unit. A reflection protection unit may be provided to prevent undesired reflection at inlet of the optical isolator. A optical filter for reflecting a component having a predetermined wavelength and transmitting remaining component may be used for the reflection unit. An optical detector may be used for monitoring the remaining component. An absorber may be used for absorbing the remaining components. A half mirror may be used for the reflection unit for monitoring the signal ray.

This is a division of application Ser. No. 08/160,787, filed Dec. 3,1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a polarization independent optical isolatorand particularly to a reflection type of a polarization independentisolator.

2. Description of the Prior Art

Polarization independent optical isolators find a wide variety ofapplications in light wave communication systems. In general, isolatorsare utilized to prevent reflected portions of a transmitted signal fromre-entering the transmitting device. Such isolators utilizes themagnetooptic effect of rotating polarized light planes unreversibly tointercept light travelling in the backward direction.

FIG. 9 is perspective view of a prior art polarization independentoptical isolator which is disclosed in U.S. Pat. No. 5,033,830. A pairof stacked reciprocal rotators, namely half-wave plates 216 and 226, aFaraday rotator 218, and reflector 224 are positioned in tandem adjacentto the birefringent plate 214. In the forward (transmitting) direction,a light wave signal exiting an optical fiber is split into a pair oforthogonal rays by the birefringent plate 214. The orthogonal rays thenpass through a first reciprocal rotator and the Faraday rotator forrotating polarizing light planes. The Faraday rotator 218 rotatespolarizing light planes 22.5 degrees. The rotated rays are thenredirected by the reflector 224 back through the Faraday rotator 218.After passing through the second reciprocal rotator, the orthogonal raysre-enter the same birefringent plate 214 where they are recombined andlaunched in an output fiber. Since a Faraday rotator is a non-reciprocaldevice, any signal traveling through the isolator in the reverse(isolation) direction will be split on both passes through thebirefringent plate 214 such that neither will intercept the input fiber.

A rate of interception of backward transmitting light, namely, theisolation characteristic depends on a characteristic of a magnetoopticcrystal (Faraday rotator). Therefore, the prior art polarizationindependent optical isolator shown in FIG. 9 has an insufficientisolation characteristic for applications to the high qualitytransmission systems or the optical fiber amplifiers. More specifically,there is only one stage of isolation structure because one stage of anisolation operation requires transmitting of a signal light through thebirefringent plate 214 twice. Generally, one stage of isolationstructure provides an isolation characteristic of about 35 dB.Therefore, the prior art polarization independent optical isolator shownin FIG. 9 has an insufficient isolation characteristic for applicationsto the high quality transmission systems or the optical fiberamplifiers.

SUMMARY OF THE INVENTION

The present invention has been developed in order to remove theabove-described drawbacks inherent to the conventional polarizationindependent optical isolator.

According to the present invention there is provided a polarizationindependent optical isolator comprising: a first birefringent plate forseparating a signal ray passing therethrough into first and second rayshaving first and second orthogonal polarizations wherein the ray of thefirst polarization experiences a first spatial displacement in a firstdirection as it passes through the first birefringent plate in a firsttransmitting direction and second spatial displacement in a second,opposite direction as it passes through the first birefringent plate ina second direction; a non-reciprocal rotating unit for rotating thefirst and second rays about π/4+n π/2 in a first rotational direction inthe first transmitting direction, wherein n=0, 1, 2, . . . and the firstrotational direction defined by the first transmission direction; asecond birefringent plate for combining the first and second rays fromthe non-reciprocal rotating unit to produce a combined ray wherein theray of the first polarization experiences third spatial displacement inthe second direction as it passes through the second birefringent platein the first transmitting direction and fourth spatial displacement inthe first direction as it passes through the second birefringent platein the second transmission direction, a distance of the first spatialdisplacement being equal to that of the third spatial displacement; afirst reciprocal rotating unit provided between the first and secondbirefringent plates so as to intercept the first and second rays forrotating the first and second rays about π/4 in the first rotationaldirection in the first transmitting direction and for rotating the raysabout π/4 in a second, opposite rotational direction in the secondtransmitting direction; a reflection unit for redirecting the combinedray back into the second birefringent plate, the second birefringentplate separating the redirected combined ray into third and fourth rays,the non-reciprocal rotating unit for rotating the third and fourth raysabout π/4+n π/2 in the first rotational direction in the secondtransmitting direction; and a second reciprocal rotating unit providedbetween the first and second birefringent plates so as to intercept thethird and fourth rays for rotating the third and fourth rays about π/4in the first rotational direction in the second transmitting directionand for rotating the rays about π/4 in the second rotational directionin the first transmitting direction, the first birefringent platecombining the third and fourth rays from the non-reciprocal rotatingunit to produce a combined output ray.

According to the present invention there is also provided a polarizationindependent optical isolator comprising: a first birefringent plate forseparating a signal ray passing therethrough into first and second rayshaving first and second orthogonal polarizations wherein the ray of thefirst polarization experiences a first spatial displacement in a firstdirection as it passes through the first birefringent plate in a firsttransmitting direction and second spatial displacement in a second,opposite direction as it passes through the first birefringent plate ina second direction; a non-reciprocal rotating unit for rotating thefirst and second rays about π/4+n π/2 in a first rotational direction inthe first transmitting direction, wherein n=0, 1, 2, . . . and the firstrotational direction defined by the first transmission direction; asecond birefringent plate for combining the first and second rays fromthe non-reciprocal rotating unit to produce a combined ray wherein theray of the first polarization experiences third spatial displacement inthe second direction as it passes through the second birefringent platein the first transmitting direction, a first distance of the firstspatial displacement being equal to that of the third spatialdisplacement; a reflection unit for reflecting the combined ray, thesecond birefringent plate separating the reflected combined ray passingtherethrough into third and fourth rays having the first and secondorthogonal polarizations wherein the ray of the first polarizationexperiences a fourth spatial displacement in the first direction as itpasses through the second birefringent plate in a second transmittingdirection, the non-reciprocal rotating unit rotating the third andfourth rays about π/4+n π/2 in the first rotational direction in thesecond transmitting direction; and a third birefringent plate forcombining the third and fourth rays from the non-reciprocal rotatingunit to produce a combined output ray wherein the ray of the firstpolarization experiences fifth spatial displacement in the firstdirection as it passes through the third birefringent plate in thesecond transmitting direction and sixth spatial displacement in thesecond direction as it passes through the third birefringent plate inthe first transmission direction, a second distance of the fourthspatial displacement being equal to that of the fifth spatialdisplacement.

According to the present invention there is further provided apolarization independent optical isolator for transmitting a signal rayin a forward direction and suppressing backward transmission comprising:a first and second optical isolation unit for receiving, transmitting,and outputting signal rays in the forward direction and for suppressbackward transmission of any rays; and a reflection unit for coupling tofirst and second optical isolation unit by directing the outputtedsignal ray from the first optical isolation unit to the second opticalisolation unit, the signal ray received by the first optical isolationunit transmitting in the opposite direction to the outputted signal rayfrom the second optical isolation unit. A reflection protection unit maybe provided to prevent undesired reflection at inlet of the opticalisolator. An optical filter for reflecting a component having apredetermined wavelength and transmitting remaining component may beused for the reflection unit. An optical detector may be used formonitoring the remaining component. An absorber may be used forabsorbing the remaining components. A half mirror may be used for thereflection unit for monitoring the signal ray.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and features of the present invention will become morereadily apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIGS. 1 and 2 are side views for showing a structure of a firstembodiment of polarization independent optical isolator with forward rayways and backward ray ways indicated respectively;

FIG. 3A is a perspective view of the optical fiber array of the firstembodiment shown in FIGS. 1 and 2;

FIG. 3B is a perspective view of a modified optical fiber array of thefirst embodiment;

FIG. 4 is a partial enlarged view of the optical isolator of the firstembodiment for showing a structure for preventing a returning ray;

FIG. 5A is a partial side view of a reflection portion of a firstmodification of the first embodiment;

FIG. 5B is a partial side view of a reflection portion of a secondmodification of the first embodiment;

FIGS. 6 and 7 are side views for showing a structure of a secondembodiment of a polarization independent optical isolator with forwardray loci and backward ray loci indicated respectively;

FIG. 8 is a partial side view for showing a modification of the secondembodiment; and

FIG. 9 is perspective view of a prior art polarization independentoptical isolator.

The same or corresponding elements or parts are designated as likereferences throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow will be described a first embodiment of this invention.

FIGS. 1 and 2 are side views for showing a structure of the firstembodiment of polarization independent optical isolator with forward rayloci and backward ray loci indicated respectively.

In FIG. 1, numerals 1 and 2 are single mode optical fibers, numeral 3 isan optical fiber array for arranging tips of the optical fibers 1 and 2in parallel and directing their optical axes in the predetermineddirection, the tips of the optical fibers 1 and 2 having input/outputfacets inclined about 8 degrees to a normal plane of the optical axis ofoptical fibers 1 and 2. Numeral 4 is a light polarizing dividing portionfor dividing incident light into orthogonal rays, that is, two linearlypolarized rays having an orthogonal relation in their polarizationplanes. The light polarizing dividing portion 4 comprises a birefringentcrystal (plate), such as a Rutile crystal. Numeral 6 is a Grin rodlensfor converting incident light thereto into collimated light at anexiting plane thereof. Numeral 5 is a triangle prism having an acuteangle of an apex angle sandwiched between the birefringent plate 4 andthe Grin rodlens 6 with its surfaces contacting with surfaces of thebirefringent plate 4 and the Grin rodlens 6.

Numeral 7 is a magnetooptic crystal (Faraday rotator or non-reciprocalrotator) provided inside a removable cylindrical magnet 8. Themagnetooptic crystal 7 unreversibly rotates polarized light planescounterclockwise about 45 degrees (π/4+n π/2, n=0, 1, 2 . . .) inresponse to a magnetic field of the magnet 8. Numeral 9 is a Grinrodlens for focusing collimated light incident thereto around an exitingfacet thereof. Numeral 10 is a rotational crystal comprising a half-waveplate for reversibly (reciprocally) rotating polarized light planes oflight incident thereto counterclockwise about 45 degrees. Numeral 11 isa rotational crystal comprising a half-wave plate 11 for reversiblyrotating polarized light planes of light incident thereto clockwiseabout 45 degrees.

Numeral 12 is a polarized light coupler for recombining two orthogonalrays in the same optical axes. The polarized light coupler 12 comprisesa birefringent plate, such as the Rutile crystal. Numeral 13 is a Grinrodlens for converting light incident thereto into a collimated light atan exiting surface thereof. Each of the Grin rodlenses 6, 9 and 13 has alarger numerical aperture than the optical fibers 1 and 2.

Therefore, light exiting an optical fibers 1 and 2 are coupled to Grinrodlenses 6, 9, and 13 with a low insertion loss. Numeral 14 is a mirrorfor reflecting almost all incident light thereto forming a reflectionmeans together with the Grin rodlens 13 for returning transmitting lightthrough the birefringent plate 12. Numerals 40 to 61 are arrows forschematically showing loci of transmitting light in an optical plane.

In the drawings, light travels in the horizontal direction of thedrawings and the polarizing directions are shown by the directions ofthe arrows as shown and are represented in a left side view on a normalplane of the optical axis of the optical isolator.

Operation of the first embodiment will be described.

FIG. 1 shows light loci in the forward transmitting condition.Non-polarized light 40 exiting the optical fiber 1 is incident to thebirefringent crystal 4 dividing the non-polarized light 40 into alinearly polarized ordinary ray 41 and a linearly polarizedextraordinary ray 42 having an orthogonal relation therebetween.Therefore, the ordinary ray transmits straight through the birefringentcrystal 4. On the other hand, a ray way of the extraordinary ray 42 isbent, so that it exits from the birefringent crystal 4 at a place on anexiting surface different from the ordinary ray. A separation distanceD1 between an exiting point of the ray 41 from the birefringent crystal4 and that of the ray 42 is determined by the kind of the birefringentcrystal 4 and a thickness T1 of the birefringent crystal 4.

The linearly polarized rays 41 and 42 are incident to the Grin rodlens 6at places slightly deviated from the optical axis of the Grin rodlens 6after transmission through the triangle prism 5. The incident rays 41and 42 to the Grin rodlens 6 are converted into approximately collimatedrays at the exiting plane of the Grin rodlens 6. The exiting rays 41 and42 from the Grin rodlens 6 are incident to the magnetooptic crystal 7.The magnetooptic crystal 7 rotates polarized light planes of lightincident thereto about 45 degrees counterclockwise non-reciprocally inresponse to the magnetic field of the cylindrical magnet 8. Therefore,rays 41 and 42 incident to the magnetooptic crystal 7 are converted intolinearly polarized rays 43 and 44 subjected to counterclockwise rotationof polarized planes about 45 degrees. The magnetooptic crystal 7 rotatesthe polarized rays 41 and 42 π/4 counterclockwise, a first rotationaldirection RD1 (a depth direction in the drawing) in the firsttransmission direction TDR1 (rightward in the drawing) and rotates thepolarized rays π/4 counterclockwise also, the first rotational directionRD1 in the second transmission direction TDR2 (leftward in the drawing).

The rays 43 and 44 exiting the magnetooptic crystal 7 enter the Grinrodlens 9. The ray 41 transmits through the Grin rodlens 6 on a upperray locus (in the drawing) against the ray 42 but the ray 43 derivedfrom the ray 41 transmits through the Grin rodlens 9 on a lower raylocus (in the drawing) against the ray 44. The Grin rodlens 9 focusesthe the rays 43 and 44 at two places slightly deviated from the opticalaxis thereof around an exiting plane of the Grin rodlens 9 such thatthese two places are point-symmetrical with respect to the center of themagnetooptic crystal 7.

The linearly polarized rays 43 and 44 exiting the Grin rodlens 9transmit the half-wave plate 10, so that polarized planes of the rays 43and 44 are subjected counterclockwise rotation of about 45 degrees, i.e.are rotated in the first rotation direction RD1. Therefore, rays exitingthe half-wave plate 10 have polarized planes different from the rays 41and 42 by 90 degrees respectively and enter the birefringent crystal 12.The birefringent crystal 12 has the same distance D2 for dividing theordinary ray and the extraordinary ray as the distance D1 of thebirefringent crystal 4 and is arranged to have the dividing of theextraordinary ray in a vertical direction opposite to that of thebirefringent crystal 4. That is, spatial displacement of theextraordinary ray passing through the birefringent crystal 12 against anordinary ray is in the first direction FD.

Therefore, a ray locus of the liner polarized ray 43 is bent because ofthe linear polarized ray 43 is an extraordinary ray such that the linearpolarized ray 43 is re-combined with the linear polarized ray 44. Therays re-combined transmits along the same ray way as a combined ray 45.The structure mentioned above forms a first stage of the opticalisolator. Here, the kind of the birefringent crystal 4 is the same asthe birefringent crystal 12 and thickness T1 of the birefringent crystal4 is the same as the birefringent crystal 12 because the same spatialdisplacements D1 and D2 are provided.

Then, the combined ray 45 enters the Grin rodlens 13 which converts thecombined ray 45 into an approximately collimated combined ray 45 at theexiting plane. The collimated combined ray 45 is reflected at thereflector 14. The reflected ray 46 enters the Grin rodlens 13 again. TheGrin rodlens 13 focuses the reflected ray on an end facet adjacent tothe birefringent crystal 12 at a place which is symmetrical to thefirstly entering place of the Grin rodlens 13 with respect to theoptical axis of the Grin rodlens 13.

The reflected ray 46 is divided into linear polarized orthogonal rays 47and 48 by the birefringent crystal 12 in accordance with direction ofpolarized planes wherein the ray of an extraordinary ray is subjectedspatial displacement in the first direction. Orthogonal rays 47 and 48transmit through the half-wave plate 11 which further rotates polarizedplanes of orthogonal rays 47 and 48 counterclockwise about 45 degreesrespectively, i.e., in the first rotational direction RD1, and then,enter the Grin rodlens 9. Here, the half-wave plate 11 rotates lightincident thereto from the right (in the drawing) counterclockwise (RD1)about 45 degrees because it rotates the polarized plane of lightincident thereto from the left (in the drawing) clockwise (RD2 which isopposite to RD1) about 45 degrees.

The magnetooptic crystal 7 rotates polarized planes of rays incidentthereto counterclockwise about 45 degrees irrespective of the incidentdirection of light, so that the linearly polarized rays 47 and 48transmits the magnetooptic crystal 7 and their polarized planes arefurther rotated counterclockwise (RD1) about 45 degrees. Therefore,polarized directions of the linearly polarized rays 49 and 50 aredifferent from the linearly polarized rays transmitted through thebirefringent crystal 12 by 90 degrees respectively.

The ray 47 transmits through the Grin rodlens 9 on a upper ray way (inthe drawing) against the ray 48 but the ray 49 derived from the ray 47transmits through the Grin rodlens 6 on a lower ray locus (in thedrawing) against the ray 50. The Grin rodlens 6 focuses the the rays 49and 50 at two places slightly deviated from the optical axis thereofaround an exiting plane of the Grin rodlens 6 such that these two placesand incident points of the ray 48 and 47 of the Grin rodlens 9 arepoint-symmetrical with respect to the center of the magnetooptic crystal7.

The ray 49 and 50 are incident to the birefringent crystal 4 aftertransmitting through the triangle prism 5. The linearly polarized ray 49is incident to the birefringent crystal 4 as an extraordinary ray, sothat the ray 49 is bent upwardly (in the drawing) and are recombinedwith the linear ray 50 on the same optical axis as a recombined ray 51.The recombined ray 51 is incident to the optical fiber 2 fixed to theoptical fiber array 3 with a low insertion loss. The above-mentionedstructure forms the second optical isolator.

Operation of the first embodiment when light enters in the oppositedirection will be described with reference to FIG. 2.

FIG. 2 shows light loci in the backward transmitting condition.Non-polarized light 52 exiting the optical fiber 2 is incident to thebirefringent crystal 4 for dividing the non-polarized light 40 into alinearly polarized ordinary ray 54 and a linearly polarizedextraordinary ray 53 having an orthogonal relation therebetween.Therefore, the ordinary ray 54 transmits straight. On the other hand, aray locus of the extraordinary ray 53 is bent, so that it exits from thebirefringent crystal 4 at a place on an exiting surface different fromthe ordinary ray 54.

The linearly polarized rays 53 and 54 are incident to the Grin rodlens 6at a places slightly deviated from the optical axis of the Grin rodlens6 after transmission through the triangle prism 5. The incident rays 53and 54 to the Grin rodlens 6 are converted into approximately collimatedrays at the exiting plane of the Grin rodlens 6. The exiting rays 53 and54 from the Grin rodlens 6 are incident to the magnetooptic crystal 7.The magnetooptic crystal 7 rotates polarized light planes of lightincident thereto counterclockwise (RD1) about 45 degreesnon-reciprocally in response to the magnetic field of the cylindricalmagnet 8. Therefore, rays 53 and 54 incident to the magnetooptic crystal7 are converted into linearly polarized rays 56 and 55 subjected tocounterclockwise rotation of polarized planes of about 45 degrees. Therays 55 and 56 exiting the magnetooptic crystal 7 enter the Grin rodlens9. The ray 54 transmits through the Grin rodlens 6 on a upper ray locus(in the drawing) against the ray 53 but the ray 56 derived from the ray54 transmits through the Grin rodlens 9 on a lower ray locus (in thedrawing) against the ray 55. The Grin rodlens 9 focuses the rays 55 and56 at two places slightly deviated from the optical axis thereof aroundan exiting plane of the Grin rodlens 9 such that these two places andincident points at the Grin rodlens 6 are point-symmetrical with respectto the center of the magnetooptic crystal 7.

The linearly polarized rays 55 and 56 exiting the Grin rodlens 9transmit the half-wave plate 11, so that polarized planes of the rays 55and 58 are subjected clockwise rotation (RD2) about 45 degrees.Therefore, rays exiting the half-wave plate 11 have the same polarizedplanes as the rays 53 and 54 respectively and enter the birefringentcrystal 12. Here, the the polarized planes of the exiting the half-waveplate 11 are different from the case shown in FIG. 1. Therefore, a raylocus of the liner polarized ray 55 is bent in the second direction SDbecause of the linearly polarized ray 55 is an extraordinary ray suchthat the linearly polarized ray 43 is not re-combined with the linearlypolarized ray 56. On the other hand, the linearly polarized ray 56transmits through the half-wave plate 11 without bending because thelinearly polarized ray 56 is ordinal ray for the birefringent crystal12. Therefore, the linearly polarized rays 55 and 56 do not enter theoptic fiber 1 basically.

Since one stage of the optical isolator mentioned above has an isolationcharacteristic of about 35 dB, so that it cannot provide a sufficientisolation characteristic. That is, there may be still a weak ray 57transmitting in the backward direction.

The non-polarized ray 57 enters the birefringent crystal 12 whichdivides the ray 57 into two linearly polarized rays 58 and 59 having anorthogonal relation therebetween, that is, into the linearly polarizedorthogonal rays 58 and 59. Linear polarized rays 58 and 59 transmitthrough the half-wave plate 10 which rotates polarized planes oforthogonal rays 58 and 59 clockwise (RD2) about 45 degrees respectivelyand then, enter the Grin rodlens 9. After transmission through the Grinrodlens 9, the linearly polarized rays 58 and 59 enter the magnetoopticcrystal 7.

The magnetooptic crystal 7 rotates polarized planes of rays incidentthereto counterclockwise (RD1) about 45 degrees irrespective of theincident direction of light, so that the linearly polarized rays 58 and59 are converted into linearly polarized rays 60 and 61.

The linearly polarized rays 60 and 61 have the same polarized directionsas the linearly polarized rays just after separation by the birefringentcrystal 12 respectively and are different from the case of the forwardtransmission shown in FIG. 1 where there is a difference of 90 degreesrespectively. Therefore, the linearly polarized ray 60 incident to thebirefringent crystal 4 after transmission of the Grin rodlens 6 is bentas an extraordinary ray in the second direction SD. On the other handthe linearly polarized ray 61 transmits straight as an ordinary ray.Accordingly, the linearly polarized rays 60 and 61 are not recombined onthe same optical axis and show optical loci different from the case ofthe forward transmission locus 40 shown in FIG. 1.

Here, an interval INT between the optical fibers 1 and 2 is set to belarger than the separation distances D1 and D2 of the extraordinary rayof the birefringent crystals 4 and 12. Therefore, the non-polarized ray57 cannot enter the optical fiber 1.

As mentioned above, the optical isolator of the first embodimenttransmits the forward ray 40 emitted from the optical fiber 1 to theoptical fiber 2 with a low insertion loss and suppresses the reverserays 52 and 57 emitted from the optical fiber 2 by two stages of theisolating operations to prevent the reverse rays 52 and 57 from enteringthe optical fiber 1. Therefore, the optical isolator of the firstembodiment functions as two stages of the polarization independentoptical isolator of the reflection type.

As mentioned above, the structure of this embodiment has features asfollows:

Two optical fibers 1 and 2, the birefringent crystal 4, the grin rodlens6, the magnetooptic crystal 7 for rotating a polarized incident raythereto about 45 degrees, the Grin rodlens 9, the birefringent crystal12, a reflection means having the grin rodlens 13 and the mirror 14 arearranged in order mentioned. Moreover, the half-wave plates 10 and 11are provided between the Grin rodlens 9 and the birefringent crystal 12at respective positions corresponding to the optical fibers 1 and 2,each of half-wave plates 10 and 11 rotating polarized planes in theopposite directions about 45 degrees. Further, the birefringent crystals4 and 12 have the same separation distances D1 and D2 between theordinal ray and the extraordinary ray and are arranged to have opticalaxes thereof in the opposite direction to provide the separationdirections of the extraordinary rays in the opposite directions.Moreover, the interval between the optical fibers 1 and 2 are set to belarger than the separation distances of the birefringent crystals 4 and12.

According to the above-mentioned structure, two stages of the opticalisolator is provided, so that the number of the parts of the opticalisolator is reduced because most of all parts are common between thesetwo stages of the optical isolators. In other words, the polarizationindependent optical isolator having a higher isolation characteristiccan be provided with a low cost and with a small size.

Moreover, the optical fibers 1 and 2 are mounted on one side of theoptical isolator having two stages of isolation structure, so that thearrangement of the optical fibers 1 and 2 is easy on assembling and aspace necessary for the optical fibers 1 and 2 can be reduced.

Further, the magnet 8 can be removed from the side of the reflector 14because two optical fibers 1 and 2 are mounted on the one side of theoptical isolator. Therefore, the magnetic field applied to themagnetooptic crystal 7 can be inverted by changing the magnet 8 toanother magnet, so that this optical isolator intercepts the incidentray from the optical fiber 1 and transmits the incident ray from theoptical fiber 2. That is, the inside of the magnet 8 has a largerdimension than any portions of the optical isolator of this invention inthe radial direction with respect to the optical axis of the opticalisolator.

In the above-mentioned embodiment, the half-wave plates 10 and 11 areprovided between the Grin rodlens 9 and the birefringent crystal 12.However, it is possible that the half-wave plates 10 and 11 are providedbetween the birefringent crystal 4 and the Grin rodlens 6 with thesimilar isolation characteristic.

FIG. 3A is a perspective view of the optical fiber array (holder) 3 ofthe first embodiment shown in FIGS. 1 and 2. The optical fibers 1 and 2are arranged in parallel and are united with array members 15.Therefore, end facets of the optical fibers 1 and 2 can be polishedtogether. Moreover, these optical fibers 1 and 2 can be assembled withthe birefringent crystal 4 at a time. Further, the facet 16 of theoptical fiber array 3 are polished to be inclined about eight degreeswith respect to a normal plane of the optical axes of the optical fibers1 and 2. Therefore, the inclined facet 16 is so arranged as to contactwith a first surface 4a of the birefringent crystal 4 to preventreflection light developed at the facet of the birefringent crystal 4from returning to the optical fibers 1 and 2. FIG. 3B is a perspectiveview of a modified optical fiber array 3' of the first embodiement. Theoptical fibers 1 and 2 are arranged in parallel and are united witharray members 15'. Therefore, end facets of the optical fibers 1 and 2can be polished together. The facet 16' is inclined also but is inclinedto the normal plane of the optical axis of the optical isolator of thisinvention in the different direction from the optical fiber array 3. Thefacet 16 or 16' of the optical fiber array may be inclined to the normalplane of the axis of the optical isolator in any direction to preventreflection light therefrom from entering the birefringent plate 4 again.

FIG. 4 is a partial enlarged view of the optical isolator of the firstembodiment for showing a structure of preventing the returning of raysat the optical fiber coupling portion. The birefringent crystal 4contacts with the inclined facet 16. An optical axis 17 of the opticalfiber array 3 is arranged to have an inclined angle of several degreesto a center axis 18 of the Grin rod lens 6. This structure preventsreflection light developed at respective interfaces through which theray emitted from the optical fiber 2 transmits from returning to theoptical fiber 1 or 2.

Moreover, the triangle prism 5 is provided with first and second sidesurfaces 5a and 5b contacting with a second surface 4b of thebirefringent crystal 4 and a surface 6a of the Grin rodlens 6. Thisprovides a surer mounting of the birefringent crystal 4 and a refractionratio matching between the birefringent crystal 4 and the Grin rodlens6. Therefore, a considerable large attenuation characteristic ofreflection light can be provided.

The birefringent crystal 4 is arranged to have the inclined angle to theoptical axis 17. However, the birefringent crystal 4 is inclined to theoptical axis within a plane including the ordinary ray 19 and theextraordinary ray 20, so that an adversary effect on the birefringentcharacteristic due to the inclined structure can be minimized.

In the first embodiment shown in FIGS. 1 and 2 has the mirror 14 as thereflection means. On the other hand, FIG. 5A shows a modification of thefirst embodiment and is a partial side view of a reflection portion. Adielectric film 21 is deposited on one end surface of the Grin rodlens13. This provides a simple structure of the reflection means.

FIG. 5B shows a second modification of the first embodiment and is apartial side view of a modified reflection portion. A half mirror 22 forreflecting a part of incident light thereto and for transmitting aremaining portion of the light, i.e., a remaining ray 62, is provided inplace of the reflector 14. A photodetector 23 is provided behind thehalf mirror 22 for detecting the remaining portion 62 of light.Therefore, the remaining ray 62 can be provided and can be monitored.

FIGS. 6 and 7 are side views of a second embodiment of an opticalisolator of this invention. FIG. 6 shows light loci of forwardtransmission. FIG. 7 shows light loci of backward transmission.

The basic structure of the second embodiment is the same as the firstembodiment. A difference of the second embodiment from the firstembodiment is as follows:

The birefringent crystal 4 is replaced with a birefringent crystals 24and 25, such as a rutile crystal, for separating incident light into twolinearly polarized rays having an orthogonal relation therebetween. Thehalf-wave plates 10 and 11 of the first embodiment is replaced with ahalf-wave plate 26. In place of the mirror 14, an optical filter(reflector) 27 for reflecting light having a predetermined wavelengthand for transmitting other wavelength light is provided. A lightabsorber 28 is provided adjacent to the optical filter 27 to receive andabsorb light transmitted through the optical filter 27. Light absorber28 can be replaced with a photodetector 23 similar to that shown in FIG.5B and further discussed herein with respect to the first embodiment.

The birefringent crystals 12, 24, and 25 have the same separationdistance between the ordinary ray and the extraordinary ray. Thebirefringent crystal 24 is so arranged that an optical axis thereof isopposite to that of the birefringent crystal 25 in order that theextraordinary ray is separated in the opposite direction as shown inFIG. 6. A half-wave plate 26 reversibly rotates a polarized plane of anincident light counterclockwise about 45°. The Grin rodlens 13, theoptical filter, the light absorber 28 forms a reflection means.

The facet 16 of the optical fiber array 3 is polished to have aninclined angle. Facets of the optical fibers 1 and 2 are arranged tocontact with respective surfaces of the birefringent crystals 24 and 25.The optical axis of the optical fiber array 3 is inclined to the centeraxis of the Grin rodlens 6. The birefringent crystals 24 and 25 areinclined within a plane including the ordinary and extraordinary raysseparated to reduce an adversary effect of birefringent effect in thecase of the inclined entrance of light. The same or correspondingelements or parts to the first embodiment are designated as likereferences and thus, a detailed description is omitted.

As mentioned above, the difference of this second embodiment from thefirst embodiment is that: in place of the birefringent crystal 4, thebirefringent crystals 24 and 25 having opposite optical axes each otherare respectively provided at positions through which light emitted fromthe optical fibers 1 and 2; in place of the half-wave plates 10 and 11,the single half-wave plate 26 are provided; in place of the reflector14, the optical filter 27 for reflecting light having the predeterminedwavelength and for transmitting the remaining ray; and the opticalabsorber 28 for absorbing the remaining ray from the optical filter 28.

Operation of the forward transmission of the second embodiment well bedescribed with reference to FIG. 6. However, the optical fiber array 3,the triangle prism 5, the magnet 8, the Grin rodlenses 6, 9, and 13 arecommon to the first embodiment. Therefore, a detailed description isomitted.

A non-polarized ray 40 emitted from the optical fiber 1 is separated bythe birefringent crystal 24 into two linearly polarized first and secondrays 41, 42 having an orthogonal relation therebetween. The linearlypolarized rays 41 and 42 are incident to the magnetooptic crystal 7.

The magnetooptic crystal 7 rotates polarized planes of the linearlypolarized rays 41 and 42 counterclockwise (RD1) 45 degrees to producelinearly polarized rays 43 and 44. The linearly polarized rays 43 and 44transmit through the Grin rodlens 9 and the half-wave plate 26. Thehalf-wave plate 26 further rotates polarized planes of the linearlypolarized rays 43 and 44 counterclockwise (RD1) 45 degrees. That is, thelinearly polarized rays after transmission of the half-wave plate 26have the polarized planes which is different from the linearly polarizedrays 41 and 42 by 90 degrees. Therefore, the linearly polarized ray 43converted from the ray 41 whose light locus was not bent as an ordinaryray by the birefringent crystal 12. On the other hand, the linearlypolarized ray 44 derived from the ray 42 whose light locus was bent asan extraordinary ray transmits straight through the birefringent crystal12. Therefore, linearly polarized rays 43 and 44 are recombined andthus, converted into a combined ray 45. The above-mentioned structureforms a first stage of optical isolator of the second embodiment.

The combined ray 45 transmits through the Grin rodlens 13 and isreflected at the optical filter 27. The reflected ray 46 enters thebirefringent crystal 12 again.

The optical filter 27 reflects a ray having a predetermined wavelengthand transmits the remaining light therethrough. Therefore, the opticalfilter 27 removes unnecessary ray 63 by transmitting therethrough. Theunnecessary ray 63 is absorbed by the absorbing member 28.

The reflected ray 46 is separated into two linearly polarized third andfourth rays 48, 47 by the birefringent crystal 12. Then, polarizedplanes of the linearly polarized rays 47 and 48 are rotated clockwise(RD2) about 45 degrees by the half-wave plate 26. The linearly polarizedrays 47 and 48 transmit through the Grin rodlens 9 and enter themagnetooptic crystal 7. The magnetooptic crystal 7 rotates the polarizedplanes of the linearly polarized rays 47 and 48 counterclockwise (RD1)about 45 degrees. Therefore, the linearly polarized rays 47 and 48 areconverted into linearly polarized rays 49 and 50. The linearly polarizedrays 49 and 50 have polarized planes having directions which is the sameas the linearly polarized rays between the birefringent crystal 12 andthe half-wave plate 26.

The birefringent crystal 25 is arranged to have the optical axis thereofin the opposite direction to the birefringent crystal 24 to obtain theseparation of an extraordinary ray in the opposite direction to thebirefringent crystal 24. Therefore, the linearly polarized ray 50 whichwas bent as an extraordinary ray by the birefringent crystal 12 is bentin the first direction FD. On the other hand, the linearly polarized ray49 transmits straight through the birefringent crystal 25. Accordinglythe linearly polarized rays 49 and 50 are combined as a combined ray 51.The combined ray 51 enters the optical fiber 2 with a low insertionloss. The above-mentioned structure forms a second optical isolator ofthe second embodiment.

Operation of the second embodiment in the backward transmission will bedescribed.

A non-polarized ray 152 emitted at the optical fiber 2 enters thebirefringent crystal 25. The birefringent crystal 25 separates thenon-polarized ray 152 into linearly polarized rays 153 and 154 having anorthogonal relation therebetween. Ray 154 is bent. The linearlypolarized rays 153 and 154 are converted into linearly polarized rays155 and 156 whose polarized planes are rotated counterclockwise about 45degrees by the magnetooptic crystal 7 after transmission through theGrin rod lens 6. Then, the linearly polarized rays 155 and 156 arefurther rotated counterclockwise about 45 degrees by the half-wave plate26 after transmission through the Grin rodlens 9.

The linearly polarized rays 155 and 156 incident to the birefringentcrystal 12 have polarized planes different from the linearly polarizedrays 53 and 54 by about 90 degrees. Therefore, the linearly polarizedray 156 transmits straight as an ordinary ray but the linearly polarizedray 155 bent as an extraordinary ray. Those light locus are differentfrom the case of the forward transmission as shown in FIG. 6. Therefore,the non-polarized ray 152 emitted by the optical fiber 2 cannot enterthe optical fiber 1.

Operation of the second embodiment in the backward transmission will bedescribed.

A ray 157 which was not intercept by the above-mentioned firsts stage ofthe optical isolator of the second embodiment and reflected by theoptical filter 27 is separated into two linearly polarized rays 158 and159 by the birefringent crystal 12 and enters the half-wave plate 26.The half-wave plate 26 rotates polarized planes of the linearlypolarized rays 158 and 159 clockwise about 45 degrees respectively. Themagnetooptic crystal 7 rotates the polarized planes counterclockwiseabout 45 degrees after transmission through the Grin rodlens 9.Therefore, linearly polarized rays 160 and 161 are obtained. Directionsof the linearly polarized rays 160 and 161 are the same as the linearlypolarized rays separated by the birefringent crystal 12. Therefore, thelinearly polarized ray 161 transmits the birefringent crystal 24straight as an ordinary ray but the linearly polarized ray 160 is bentas an extraordinary ray. That is, the linearly polarized rays 160 and161 transmit through the birefringent crystal 27 in the different mannerof the forward transmitting, so that the non-polarized ray 157 cannotenter the optical fiber 1.

As mentioned above, the optical isolator of the second embodimenttransmits a light signal from the optical fiber 1 to the optical fiber 2and prevents the ray from the optical fiber 2 from entering the opticalfiber 1 through two stages of optical isolation structures.

In the second embodiment, birefringent crystals 24 and 25 are arrangedto have optical axes in opposite directions therebetween at respectivepositions through which rays emitted from the optical fibers 1 and 2.Moreover, a single half-wave plate 26 which commonly used for the firstand second stages is provided. The birefringent crystals 12, 24, and 25have the same separation distance between the ordinary ray and theextraordinary ray and an interval of the optical fibers 1 and 2 is setto be larger than the separation distance. The birefringent crystals 24and 25 are arranged to have optical axes in opposite directionstherebetween.

This structure provides a small size configuration and a favorableisolation characteristic with the number of parts reduced at a low costbecause there are two stages of optical isolation operations. Moreover,the reflection means of this embodiment is formed by the Grin rodlens13, optical filter 27, and the absorbing member 28. This structure canremove the unnecessary ray and absorbs the unnecessary ray.

FIG. 8 is a partial side view for showing a modified embodiment of thesecond embodiment. In place of the absorbing member 28, a Grin rodlens29 is provided for receiving the remaining ray 64 from the opticalfilter 27 and an optical fiber 30 is provided for receiving theremaining ray 64 from the Grin rodlens 29.

The rodlens 29 receives and transmits the remaining ray from the opticalfilter 27 toward the optical fiber 30. The optical fiber 30 receives theremaining ray 64 from the Grin rodlens 29. This structure provides anoptical separation in accordance with the predetermined wavelength ofthe optical filter 27. The position of the optical fiber 30 is set toreceive the remaining ray 64 coming from the optical fiber 1. However,it is possible to receive the ray coming from the optical fiber 2 havinga wavelength other than the predetermined wavelength by selection of theposition of the optical fiber 30.

In the second embodiment, the half-wave plate 26 is provided between theGrin rodlens 9 and the birefringent crystal 12. However, the half-waveplate 26 can be provided between the Grin rodlens 6 and the birefringentcrystals 24 and 25.

In the first and second embodiments, Grin rodlenses 6, 9, 13, and 29 areused. However, spherical lenses or aspherical lenses having largenumerical apertures can be used. In this case, aberrations at positionsdeviated from the center axis of the spherical lens or the asphericallens should be compensated. Moreover, the grin rod lenses 6, 9, 13, and29 or spherical lens, that is, any lens can be omitted from the opticalisolators of the first and second embodiments as similar to the priorart shown in FIG. 9.

In the first and second embodiment, a description about the spacebetween respective optical elements is omitted. The space may be the airor be filled with other transparent material, such as refractive indexmatching material. Further, at interface surfaces of respective opticalelements, anti-reflection coatings may be formed to reduce unnecessaryreflection in the optical isolator.

The modifications of the first embodiment shown in FIGS. 5A and 5B arealso applicable to the second embodiment. Further, the modification ofthe second embodiment shown in FIG. 8 is also applicable to the firstembodiment.

What is claimed is:
 1. A polarization independent optical isolatorcomprising:(a) a first birefringent plate for separating a signal raypassing therethrough into first and second rays having first and secondorthogonal polarizations wherein the first ray of the first orthogonalpolarization experiences a first spatial displacement as it passesthrough said first birefringent plate in a first transmitting direction;(b) non-reciprocal rotating means for rotating said first and secondrays about π/4+n π/2 in a first rotational direction of said firsttransmitting direction, wherein n is an integer; (c) a secondbirefringent plate for combining said first and second rays from saidnon-reciprocal rotating means to produce a combined ray wherein thefirst ray experiences a second spatial displacement as it passes throughsaid second birefringent plate in said first transmitting direction, adistance of said first spatial displacement being substantially equaland opposite in directing to that of said second spatial displacement;(d) first reciprocal rotating means provided between said first andsecond birefringent plates so as to intercept said first and second raysfor rotating said first and second rays about π/4 in a second rotationaldirection of said first transmitting direction that is opposite to saidfirst rotational direction of said first transmitting direction; (e)reflection means for redirecting said combined ray in a secondtransmitting direction into said second birefringent plate, said secondbirefringent plate separating said redirected combined ray into thirdand fourth rays, said non-reciprocal rotating means rotating said thirdand fourth rays about π/4+n π/2 in a first rotational direction of saidsecond transmitting direction; and (f) second reciprocal rotating meansprovided between said first and second birefringent plates so as tointercept said third and fourth rays for rotating said third and fourthrays about π/4 in a second rotational direction of said secondtransmitting direction that is opposite to said first rotationaldirection of said second transmitting direction; wherein said firstbirefringent plate combines said third and fourth rays from saidnon-reciprocal rotating means to produce a combined output ray; andwherein said first reciprocal rotating means is located adjacent to saidsecond reciprocal rotating means.
 2. A polarization independent opticalisolator as claimed in claim 1, further comprising:a first lens providedbetween said first birefringent plate and said non-reciprocal rotatingmeans for focusing said first and second rays and for collimating saidthird and fourth rays.
 3. A polarization independent optical isolator asclaimed in claim 2, further comprising:a second lens provided betweensaid non-reciprocal rotating means and said second birefringent platefor collimating said first and second rays and for focusing said thirdand fourth rays.
 4. A polarization independent optical isolator asclaimed in claim 3, further comprising:an optical fiber holder forholding a first optical fiber for emitting said signal ray to directsaid signal ray toward said first birefringent plate with a predetermineincident angle and for holding a second optical fiber to receive saidcombined output ray, said optical fiber holder holding said first andsecond optical fibers such that said first and second optical fibersarranged in parallel with a predetermined interval.
 5. A polarizationindependent optical isolator as claimed in claim 4, wherein saidpredetermined interval is larger than said first distance.
 6. Apolarization independent optical isolator as claimed in claim 4, furthercomprising:said first optical fiber for emitting and directing saidsignal toward said first birefringent plate; and said second opticalfiber for receiving said combined output ray.
 7. A polarizationindependent optical isolator as claimed in claim 6, wherein said firstand second lenses comprise first and second Grin rodlenses having largernumerical apertures than said first and second optical fibersrespectively.
 8. A polarization independent optical isolator as claimedin claim 7, wherein said optical fiber holder is arranged to have aninclined angle to said first transmitting direction.
 9. A polarizationindependent optical isolator as claimed in claim 7, wherein said opticalfiber holder has a surface confronting said first birefringent plate,said surface is a predetermined angle inclined to a normal plane of anoptical axis of said polarization independent optical isolator, saidfirst and second optical fibers having first and second facets definedby said surface.
 10. A polarization independent optical isolator asclaimed in claim 9, wherein said first birefringent plate has first andsecond parallel surfaces and are slantwise arranged such that said firstsurface contacting with said surface of said optical fiber holder isarranged to have an inclined angle to said optical axis.
 11. Apolarization independent optical isolator as claimed in claim 10,wherein said birefringent plate slantwise arranged in a plane includingsaid first and second rays transmitted therethrough.
 12. A polarizationindependent optical isolator as claimed in claim 10, further comprisingan acute triangle prism, having first and second surfaces, providedbetween said first birefringent plate and said first lens with saidsecond surface of said first birefringent plate contacting with saidfirst surface of said acute triangle prism and said second surface ofsaid acute triangle prism contacting with a surface of said first lens.13. A polarization independent optical isolator as claimed in claim 1,wherein said non-reciprocal rotating means comprises a magnetoopticcrystal and a magnet, said magnetooptic crystal non-reciprocally rotatessaid first to fourth rays in response to a magnetic field generated bysaid magnet.
 14. A polarization independent optical isolator as claimedin claim 13, wherein said magnet has a cylindrical shape and arrangedaround said magnetooptic crystal.
 15. A polarization independent opticalisolator as claimed in claim 13, wherein an inside of said magnet has adimension such that said magnet is removable from said polarizationindependent optical isolator on either side of said first birefringentplate or said reflection means.
 16. A polarization independent opticalisolator as claimed in claim 1, wherein said reflection means comprisesa third lens for collimating said combined ray and a reflector forreflecting, said third lens focusing said reflected combined ray.
 17. Apolarization independent optical isolator as claimed in claim 16,further comprising: an optical fiber holder for holding a first opticalfiber for emitting said signal ray to direct said signal ray toward saidfirst birefringent plate with a predetermined incident angle and forholding a second optical fiber to receive said combined output ray, saidoptical fiber holder holding said first and second optical fibers suchthat said first and second optical fibers such that said first andsecond optical fibers are arranged in parallel with a predeterminedinterval.
 18. A polarization independent optical isolator as claimed inclaim 17, wherein said third lens comprises a third Grin rodlens havinga larger numerical aperture than said first and second optical fibers.19. A polarization independent optical isolator as claimed in claim 18,wherein said reflector comprises a dielectric film deposited on onesurface of said third Grin rodlens.
 20. A polarization independentoptical isolator as claimed in claim 16, wherein said reflectorcomprises a half mirror.
 21. A polarization independent optical isolatoras claimed in claim 20, further comprising a detection means fordetecting light transmitted through said half mirror.
 22. A polarizationindependent optical isolator as claimed in claim 16, wherein saidreflector comprises an optical filter for reflecting a component havinga predetermined wavelength in said collimated combined ray andtransmitting the remaining portion of said collimated combined ray. 23.A polarization independent optical isolator as claimed in claim 22,further comprising detection means for detecting a remaining portion ofsaid collimated combined ray.
 24. A polarization independent opticalisolator as claimed in claim 22, further comprising an absorbing meansfor absorbing said remaining portion of said collimated combined ray.25. A polarization independent optical isolator as claimed in claim 1,wherein said first reciprocal rotating means comprises a halfwave plate.26. A polarization independent optical isolator as claimed in claim 1,wherein said second reciprocal rotating means comprises a halfwaveplate.